The concept of set: certain and distinguishable things in a certain range, as a whole, are called sets, called elements or elements of sets.

Three characteristics of a set: disorder, certainty and mutual difference.

1 . a & gt； 0, and the solution set is R.

A<0, the solution set is (xlx =-2a/bxr).

This problem needs to be combined with the function f(x)=ax? The image solution of +bx+c, which is a parabola with only one intersection with the x axis.

Since the topic says that it is a quadratic equation with one variable, then A can't be 0, so:

A>0, the image opening is upward.

When a< is 0, the opening is downward.

2.

At most one of (1) is one or none, so δ = 0 and δ.

② at least one, that is, one or two, so that δ = 0 and δ >; 0 solution is enough.

Pure hands, I hope it helps.